fix: return queue-not-found for missing queue items

[lsaether]

Summary

• Add a dedicated -32004 JSON-RPC error for missing amux/unqueue_prompt queue items.
• Keep malformed queueItemId params on -32602, so clients can distinguish invalid input from queue remove/submit races.
• Cover the missing-item response with a WebSocket server regression test.

Scope

• Single bullet from amux active-turn controls: small follow-ups from PR #41 review #43 only: missing queueItemId error semantics for amux/unqueue_prompt.
• No docs, API shape, auth, steering, or lifecycle follow-ups included.

Tests

• RED: cargo test amux_unqueue_prompt_missing_item_uses_queue_not_found_error -- --nocapture failed with -32602 before the implementation.
• GREEN: cargo test amux_unqueue_prompt_missing_item_uses_queue_not_found_error -- --nocapture
• cargo fmt --check
• git diff --check
• cargo test
• cargo clippy --all-targets -- -D warnings

Part of #43.